1. Field of the Invention
The present invention relates to an image forming apparatus, for example, an image forming apparatus of an electrophotographic system.
2. Description of Related Art
An example of a conventional image forming apparatus will be described with reference to FIGS. 12 and 13.
FIG. 12 is a view illustrating motors and an image forming part. FIG. 13 is a view illustrating a DC brushless motor and its vicinities.
FIG. 12 shows a multicolor image forming apparatus provided with motors and rotation bodies for four colors, namely, yellow Y, magenta M, cyan C, and black K. In FIG. 12, reference symbols 1Y, 1M, 1C and 1K denote the rotation bodies such as photosensitive drums functioning as developing units for forming and developing electrostatic latent images (1Y, 1M, 1C and 1K denote the rotation bodies for Y, M, C and K, respectively), and reference symbols 6Y, 6M, 6C and 6K denote the motors for driving the rotation bodies 1Y, 1M, 1C and 1K, respectively.
Reference symbols 2Y, 2M, 2C and 2K denote laser scanners for performing exposure in response to an image signal to form electrostatic latent images on the rotation bodies 1Y, 1M, 1C and 1K. Numeral 3 donates an endless conveyor belt for sequentially conveying a sheet to the rotation bodies 1Y, 1M, 1C and 1K for each color. Numeral 4 denotes driving rollers that consist of motors, gears, and the like and are connected to a driving means to drive the conveyor belt 3. Numeral 6e denotes a motor for driving the driving rollers 4. Numeral 5 denotes a fixing device for melting and fixing toner transferred onto a sheet. Numeral 15 denotes motors and an image forming part.
Data to be printed is sent to a printer from a personal computer (PC). When image formation according to a system of a printer engine ends and a printer comes to be in a printable state, a sheet is fed from a sheet cassette and reaches the conveyor belt 3 to be conveyed one after another.
An image signal of each color is sent to each of the laser scanners 2Y, 2M, 2C and 2K with taking timing with the conveyance of the sheet by the conveyor belt 3. Electrostatic latent images are formed on the rotation bodies 1Y, 1M, 1C and 1K. The electrostatic latent images are developed with toner by a developing device (not shown) and transferred onto the sheet in a transferring part (not shown).
In FIG. 12, images are sequentially formed in the order Y, M, C and K.
Thereafter, the sheet is separated from the conveyor belt 3, toner images are fixed thereon by heat in the fixing device 5, and the sheet is discharged to the outside (i.e., is discharged from the apparatus).
For example, a DC brushless motor is used as each of the motors 6Y, 6M, 6C, 6K and 6e. 
An example of a configuration of the DC brushless motor is shown in FIG. 13.
Reference numeral 51 denotes coils; numeral 52 denotes a rotor; numeral 53 denotes Hall elements; numeral 54 denotes an amplifier; numeral 55 denotes a magnetic pattern; numeral 56 denotes a magnetic sensor; numeral 57 denotes an amplifier; numeral 58 denotes a current logic circuit for controlling current; numeral 60 denotes a rate control part; numeral 61 denotes an F/V converter; numeral 62 denotes a comparator; numeral 63 denotes a PLL; numeral 64 denotes a mixer; numeral 65 denotes a PWM signal generator; numeral 70 denotes a driver; numeral 71 denotes high-side transistors; numeral 72 denotes low-side transistors; numeral 80 denotes a current limiter; numeral 81 denotes a current detection resistor; and numeral 82 denotes a comparator. Reference symbols HU, HV and HW denote rotor position signals, and UU, UV and UW, and LU, LV and LW denote phase switching signals.
The DC brushless motor 50 has the U, V and W coils 51 connected in three-phase star connection and the rotor 52.
Moreover, the DC brushless motor 50 is provided with three Hall elements 53 for detecting polarity of the rotor as a position detecting means of the rotor 52. Outputs of the Hall elements are amplified by the amplifier 54.
In addition, the DC brushless motor 50 has a rotation rate detecting means consisting of the magnetic pattern 55 and the magnetic sensor 56 that are provided on an outer circumference of the rotor 52. An output of the rotation rate detecting means is amplified by the amplifier 57 and inputted in the rate control part 60.
Reference numeral 70 denotes a driver for driving the DC brushless motor 50 and is provided with three high-side transistors 71 and three low-side transistors 72, which are connected to the U, V and W coils 51, respectively.
The current logic circuit 58 specifies a position of the rotor 52 in response to the rotor the position signals HU to HW generated by the Hall elements 53 and generates the phase switching signals UU, UV and UW, and LU, LV and LW.
The phase switching signals UU, UV and UW, and LU, LV and LW sequentially switch phases for controlling on/off of each of the transistors 71 and 72 of the driver 70 to sequentially switch a phase to be excited and rotate the rotor 52.
The rate control part 60 generates a voltage proportional to the number of motor rotations by the F/V converter 61 and, then, compares the voltage with a reference voltage by the comparator 62 to obtain a differential output of the voltages.
In addition, the rate control part 60 compares phases of a motor rotation frequency signal and a reference frequency signal by the PLL 63 to obtain an output according to phase shift.
Moreover, the rate control part 60 mixes these two outputs by the mixer 64 and generates a PWM signal by the PWM signal generator 65.
In the multicolor image forming apparatus configured as described above, misalignment of a print position of each color appears on an image as color drift and causes deterioration of image quality.
Color drift is generally classified into steady color drift caused by position deviation at the time of assembling a developing device of each color, or the like (hereinafter referred to as DC color misregistration), and periodic color drift caused by swinging of the shafts of the rotation bodies 1Y, 1M, 1C and 1K, or the like (hereinafter referred to as AC color misregistration).
As a measure for coping with the AC color drift, there is known a method of individually controlling rotation phases of the rotation bodies 1Y, 1M, 1C and 1K for each color. As a method of adjusting phases of the rotation bodies 1Y, 1M, 1C and 1K, for example, the technique disclosed in Japanese Patent Application Laid-Open No. 9-146329 is proposed.
With such a technique, rotary encoders which are capable of outputting many signals in one rotation and are capable of outputting one signal in one rotation, are provided in the rotation shafts of the rotation bodies 1Y, 1M, 1C and 1K in order to control the rotation phases of the rotation bodies 1Y, 1M, 1C and 1K. The rotation phases of the rotation bodies 1Y, 1M, 1C and 1K are detected by outputs of the rotary encoders and are used to control driving of the DC brushless motor idly or at a rate higher or lower than a normal rotation rate, whereby the phases of the rotation bodies 1Y, 1M, 1C and 1K are adjusted.
However, there are the following problems in the above-mentioned conventional example.
The rotary encoder which is capable of outputting many signals in one rotation and is capable of outputting one signal in one rotation, is expensive. This increases costs particularly in an image forming apparatus provided with the rotation bodies 1Y, 1M, 1C and 1K for four colors.
In addition, due to the idle rotation of the motor, a braking function for quickly stopping the motor at a point in predetermined lead phase is necessary in order to perform phase control. A rate variable function is required in order to rotate the motor at a rate higher or lower than a normal rotation rate. Both of these requirements cause increases in the cost of the motor.
However, even if improvement of resolution of phase control is attempted, high accuracy position control of the motor is difficult, and effects of the phase control cannot be sufficiently obtained with such a control method.
In particular, in a printer for which a reduction in cost is required, this control method has relatively poor cost performance and is not suitable for practical use.
In addition, since each of the rotation bodies 1Y, 1M, 1C and 1K has a different load torque, the time required for reaching a steady rate or time required for stopping is different for each of the rotation bodies 1Y, 1M, 1C and 1K. Thus, a rotation phase is likely to shift significantly every time startup and stop of the motor are repeated.
Consequently, a longer time is required for aligning a phase, and first print time is extended if the start of a print operation is delayed until phase adjustment ends.
When a stepping motor is used instead of the DC brushless motor, phase control can be performed relatively easily. However, since the mechanical efficiency of the stepping motor is low, power consumption is large. Therefore, the multicolor image forming apparatus using a plurality of motors needs a large-capacity power supply, which causes an increase in size and costs.